STM32F746-DISCo - How to grab a screen area and save this on bitmap structure ?

In order to create a faster scrollbar function I need to fill a bitmap structure with an area get from the screen.

I have starting to develop this new function starting from the BSP_LCD_DrawBitmap source but how to reverse the LL_ConvertLineToARGB8888() ?

Here the status of this develop.

void BSP_LCD_GrabBitmap(uint32_t Xpos, uint32_t Ypos, uint32_t Width, uint32_t Height, uint8_t *pbmp)
{
 uint32_t index = 0, width = 0, height = 0, bit_pixel = 0;
 uint32_t address;
 uint32_t input_color_mode = 0;
 
 grab_image.fileHeader.bfType = 0x4d42u;
 grab_image.fileHeader.bfSize = sizeof(grab_BITMAPINFOHEADER) + sizeof(grab_BITMAPFILEHEADER) + (1288 * 2);
 grab_image.fileHeader.bfOffBits = sizeof(grab_BITMAPINFOHEADER) + sizeof(grab_BITMAPFILEHEADER);
 grab_image.infoHeader.biSize = sizeof(grab_BITMAPINFOHEADER);
 grab_image.infoHeader.biWidth = Width;
 grab_image.infoHeader.biHeight = Height;
 grab_image.infoHeader.biPlanes = 1u;
 grab_image.infoHeader.biBitCount = 16;
 
 
 
 /* Get bitmap data address offset */
 //index = pbmp[10] + (pbmp[11] << 8) + (pbmp[12] << 16) + (pbmp[13] << 24);
 index = grab_image.fileHeader.bfOffBits;
 
 /* Read bitmap width */
 //width = pbmp[18] + (pbmp[19] << 8) + (pbmp[20] << 16) + (pbmp[21] << 24);
 width = grab_image.infoHeader.biWidth;
 
 /* Read bitmap height */
 //height = pbmp[22] + (pbmp[23] << 8) + (pbmp[24] << 16) + (pbmp[25] << 24);
 height = grab_image.infoHeader.biHeight;
 
 /* Read bit/pixel */
 //bit_pixel = pbmp[28] + (pbmp[29] << 8);
 bit_pixel = grab_image.infoHeader.biBitCount;
 
 /* Set the address */
 address = hLtdcHandler.LayerCfg[ActiveLayer].FBStartAdress + (((BSP_LCD_GetXSize()*Ypos) + Xpos)*(4));
 
 /* Get the layer pixel format */
 if ((bit_pixel/8) == 4)
 {
 input_color_mode = CM_ARGB8888;
 }
 else if ((bit_pixel/8) == 2)
 {
 input_color_mode = CM_RGB565;
 }
 else
 {
 input_color_mode = CM_RGB888;
 }
 
 /* Bypass the bitmap header */
 pbmp += (index + (width * (height - 1) * (bit_pixel/8)));
 
 /* Convert picture to ARGB8888 pixel format */
 for(index=0; index < height; index++)
 {
 /* Pixel format conversion */
 >>> LL_ConvertLineToARGB8888((uint32_t *)pbmp, (uint32_t *)address, width, input_color_mode);
 // to convert to ARGB8888 to Line
 
 /* Increment the source and destination buffers */
 address+= (BSP_LCD_GetXSize()*4);
 pbmp -= width*(bit_pixel/8);
 }
}